1. (Field of the Invention)
The present invention relates to a dielectric composition that can be favorably used in high-frequency regions and, particularly, in microwave regions and that can be fired at a low temperature. More particularly, the invention relates to a dielectric ceramic composition that is suited for use as an electronic part having internal conductors such as resonator, capacitor, filter and substrates incorporating them, and relates to a multilayer resonator made of the above-mentioned composition as well as to a multilayer filter using the above-mentioned resonator.
2. (Description of the Prior Art)
A variety of dielectric ceramics have heretofore been widely used as dielectric materials for electronic parts having internal conductors such as resonators, capacitors, filters and substrates containing them. Accompanying the development and widespread use of high-frequency devices such as mobile communication equipment as represented by portable telephones in recent years, the dielectric ceramics have been positively utilized as electronic parts for use in high-frequency regions.
In order that the dielectric ceramics and the internal conductors can be fired together (cofired), the internal conductors that are printed have been composed of a metal such as Pt, Pd, W, Mo or the like having a melting point higher than the firing temperature of the dielectric ceramics such as alumina, steatite, forsterite or the like, so that the internal conductors will not be melted at the firing temperature of the dielectric ceramics.
However, the above-mentioned metals have such large resistances that the electronic parts exhibit large Q-values in the resonance circuits leaving a problem of large transmission losses.
In an attempt to solve such a problem, there have been proposed a variety of dielectric ceramics using, as a conductor, a metal such as Ag or Cu having small resistance and that can be fired at low temperatures. In order to meet the demand for realizing high-frequency electronic circuit substrates in small sizes maintaining high performance, furthermore, a variety of composite dielectrics have been proposed according to which a high dielectric constant .epsilon.r is exhibited in a particular frequency region to reduce the sizes of the resonator circuit and the filter. Moreover, the dielectric ceramics have been designed to exhibit large Q-values, such that the resonator circuit and filter exhibit large Q-values, too, enabling losses to be decreased.
Japanese Laid-Open Patent Publication No. 292460/1992 discloses a dielectric ceramic composition comprising an anorthite (CaO.multidot.Al.sub.2 O.sub.3 .multidot.2SiO.sub.2)-calcium titanate glass and TiO.sub.2. This dielectric ceramic composition can be fired at such a low temperature that a metal such as Ag or Cu can be fired as a conductor at the same time.
With the dielectric ceramic composition disclosed in Japanese Laid-Open Patent Publication No. 292460/1992, however, the dielectric constant .epsilon.r is as low as 16 or smaller as measured in a high-frequency region of from 4 to 6 GHz, imposing limitation on decreasing the size of the high-frequency electronic parts.
Furthermore, the dielectric ceramic composition has a Q-value of as low as about 330 at a frequency of 6 GHz and, hence, a resonance circuit exhibits a small Q-value, too.
A ceramic composition of a three-component system, i.e., MgO--CaO--TiO.sub.2 has been widely known having a low dielectric loss (high Q-value) and a small temperature coefficient of dielectric constant.
This composition exhibits excellent dielectric properties, e.g., a dielectric constant of about 20, a Q-value of about 8000 at 7 to 8 GHz and a temperature coefficient .tau.f of resonance frequency of nearly 0.
However, this composition has a firing temperature of not lower than 1300.degree. C. and makes it difficult to use Ag (melting point, 962.degree. C.) as an internal conductor.
There has further been known a composition obtained by blending the above-mentioned MgO--CaO--TiO.sub.2 three component system with boron or a boron-containing compound.
When the MgTiO.sub.3 --CaTiO.sub.3 system is blended with boron or the boron-containing compound only, however, the firing temperature is not lowered to a sufficient degree when the blending amount is small, and the composition is not sintered at a temperature which is lower than the melting temperature of Ag or the like. When the blending amount is too large, on the other hand, the firing temperature is lowered but boron or the boron-containing compound reacts with the MgTiO.sub.3 --CaTiO.sub.3 system at a temperature of the firing. When the blending amount is too large, therefore, the unreacted MgTiO.sub.3 --CaTiO.sub.3 remains in a small amount after the firing, making it difficult to maintain a high Q-value of not smaller than, for example, 500.
With the conventional composition comprising MgO--CaO--TiO.sub.2 -boron or boron compound only, therefore, it is difficult to obtain a ceramic composition for electronic parts having low firing temperature and excellent dielectric properties in high-frequency regions.